Definition of b.i.d. (on prescription)

b.i.d. (on prescription): Seen on a prescription, b.i.d. means
twice (two times) a day. It is
an abbreviation for "bis in die" which in Latin means twice
a day. The
abbreviation b.i.d. is sometimes written without a period
either in
lower-case letters as "bid" or in capital letters as
"BID". However it is
written, it is one of a number of hallowed abbreviations of
Latin terms that
have been traditionally used in prescriptions to specify
the frequency with
which medicines should be taken.

Other examples include:

q.d. (qd or QD) is once a day; q.d. stands for "quaque
die" (which means,
in Latin, once a day).

t.i.d. (or tid or TID) is three times a day ; t.i.d.
stands for "ter in
die" (in Latin, 3 times a day).

q.i.d. (or qid or QID) is four times a day; q.i.d.
stands for "quater in
die" (in Latin, 4 times a day).

q_h: If a medicine is to be taken every so-many hours,
it is written "q_h"; the "q" standing for "quaque" and the
"h" indicating the number of hours. So, for example, "2
caps q4h" means "Take 2 capsules every 4 hours."